Polyolefin films such as polyethylene and polypropylene are commonly employed packaging materials because of their relative low cost, heat sealability and moisture resistance. However, these polyolefins have a fairly high permeability to gases including oxygen. These polyolefins are not adequate for packaging oxygen sensitive foods and other materials that degrade in the presence of oxygen or other atmospheric gases.
It is also known that polvinyl alcohol (PVOH) and its copolymers have excellent resistance to permeation of gases, particularly oxygen. However, they involve a drawback that their oxygen gas barrier property is impaired to a great extent due to moisture absorption under high relative humidity conditions. In addition, they are easy to dissolve in boiling water. Moreover, they do not bond well to many polymer films, particularly polyolefin films.
A number of methods have been known to increase the water resistance of PVOH and its copolymers. None of these methods provided entirely satisfactory results inspite of the various methods such as crosslinking PVOH with potassium dichromate, formaldehyde or phenol formaldehyde resins. PVOH still tends to soften, swell and lose adhering strength when exposed to water or even moisture vapor.
Knoerzer, U.S. Pat. No. 5,230,963 discloses oxygen and water vapor transmission resistant film comprising a maleic acid anhydride modified polypropylene films coated with aqueous polyvinyl alcohol containing melamine fomaldehyde as a crosslinking agent.
Steiner et al., U.S. Pat. No. 4,214,039 is directed to a polypropylene film having a primer comprising water dispersed epoxy resin coating and a water soluble amine modified acrylic resin applied to the treated oriented polypropylene, topcoated with an aqueous dispersion of vinylidene chloride multipolymer and subsequently dried to produce a coated packaging film.
Bianchini et al., U.S. Pat. No. 5,521,338 disclose barrier film structures comprising polyvinyl alcohol as an oxygen barrier with an aldehyde containing crosslinking agent in the presence of a catalytic amount of sulfuric acid and coated with a moisture barrier comprising polyvinylidene chloride. However, chlorine gas which is attributable to the chlorine in polyvinylidene is generated upon its incineration in waste treatment.
Markiewicz, U.S. Pat. No. 4,927,689 is directed to gas barrier structures which comprises a thermoplastic polymeric layer having two coatings on one side of the base layer. The first coating is a solvent based two component urethane primer and the second coating comprising polyvinyl alcohol as a barrier material. A drawback involves the use of organic solvent which is an environmental hazard.
There remains a need for an oxygen and moisture resistant film structure which overcomes the drawbacks of the prior art. Surprisingly, the oxygen and moisture resistant film structures of the present invention exhibit excellent oxygen gas barrier property under low relative humidity, as well as high relative humidity conditions, improved bond strength, moisture resistance properties and storage stability as compared to other film structures.